S1 Excited State Research Articles

Effects of the cyano substitution at different positions on the ESIPT properties of alizarin: A DFT/TD-DFT investigation

Three novel molecules (AL-1, AL-2 and AL-3) were designed based on the substitution of cyano group for hydrogen atoms at different positions on alizarin, and the excited state intramolecular proton transfer (ESIPT) properties of alizarin and its derivatives in ethanol were investigated systematically via density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The primary parameters associated with the ESIPT process were obtained to analyze the ESIPT properties of alizarin and its derivatives. The results show that all the intramolecular hydrogen bonds O1H1⋯O2 for alizarin and its derivatives in ethanol have been enhanced in the S1 excited state, and the intramolecular hydrogen bond O1H1⋯O2 for AL-2 in S1 state is the strongest among alizarin and its derivatives. From the obtained potential energy curves, it is shown that AL-2 exhibits the lowest potential barrier in S1 state among alizarin and its derivatives, implying that the ESIPT is more likely to occur in AL-2.

Substituent position effects on sunscreen photodynamics: A closer look at methyl anthranilate

Towards the development of a bottom-up rationale for sunscreen design, the effects of substituent position on the ultrafast photodynamics of the sunscreen precursor methyl anthranilate (MA, an ortho compound) were evaluated by studying para- and meta-MA in vacuum. Time-resolved ion yield (TR-IY) measurements reveal a long-lived S1 excited state (≫1.2 ns) for para-MA, proposed to be the result of a weakly fluorescent, bound excited state. In the case of meta-MA, TR-IY transients reveal a much faster (∼2 ns) excited state relaxation, possibly due to multiple low-lying S1/S0 conical intersections of prefulvenic character. While meta-MA may not be an ideal sunscreen ingredient due to a low ultraviolet absorbance, its comparatively efficient relaxation mechanism may constitute an alternative to common sunscreen relaxation pathways. Thus, our results should prompt further studies of prefulvenic relaxation pathways in potential sunscreen agents.

Nonadiabatic dynamics simulations on internal conversion and intersystem crossing processes in gold(i) compounds.

The position at which the second gold(i)-phosphine group is attached was experimentally found to play a noticeable role in intersystem crossing rates of gold(i) naphthalene derivatives. However, the physical origin is ambiguous. Herein we have employed generalized trajectory-based surface-hopping dynamics simulations to simulate the excited-state relaxation dynamics of these gold(i) naphthalene compounds including both the intersystem crossing process from the initially populated first excited singlet states S1 to triplet manifolds and internal conversion processes within these triplet states. Our predicted intersystem crossing rates are consistent with experiments very well. On the basis of the present results, we have found that (1) ultrafast and subpicosecond intersystem crossing processes are mainly caused by small energy gaps and large spin-orbit couplings between S1 and Tn; (2) adding the second gold(i)-phosphine group does not increase spin-orbit couplings between S1 and Tn but decrease their values remarkably, which implies that heavy-atom effects are state-specific, not state-universal; (3) the position at which the second gold(i)-phosphine group is attached has a remarkable influence on the electronic structures of S1 and Tn and their relative energies, which affect energy gaps and spin-orbit couplings between S1 and Tn and eventually modulate intersystem crossing rates from S1 to Tn. These new insights are very useful for the design of gold-containing compounds with excellent photoluminescence properties. Finally, this work also exemplifies that different isomers of a compound could have distinct excited-state relaxation dynamics.

Insights into Excited State Intramolecular Proton Transfer: An Alternative Model for Excited State Proton Transfer of Green Fluorescence Protein

The excited state intramolecular proton transfer (ESIPT) that occurs in the o-sulfonamide analogue ( o-TsABDI) of the green fluorescent protein (GFP) chromophore provides an alternative model to get insights into the excited state proton transfer (ESPT) related photophysics of GFP. In this article, we explored the ESIPT-related photophysics of o-TsABDI by electronic absorption and fluorescence emission spectra in a wide polarity range of solvents, cis- trans photoisomerization experiment, and Coulomb-attenuating method (CAM)/time-dependent (TD) density functional theory (DFT) calculations. We found that the whole ESIPT process involves four steps. The first step is photoexcitation of o-TsABDI, which does not involve charge transfer (CT). The second step is ESIPT and accompanying electron transfer from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety. The third step is fluorescence emission of the zwitterionic o-TsABDI and accompanying CT from the π* orbital of the 4-benzylideneimidazolone moiety to the half-filled n orbital of the sulfonamide nitrogen. The last step involves irreversible and barrierless proton recombination. In contrast to the isolated GFP chromophore and its p- and m-amino analogues, the S1 excited state of o-TsABDI does not relax by way of cis- trans photoisomerization through the S1/S0 conical intersection CI(I) by rotating around the I-bond, but follows the ESIPT pathway. The low fluorescence quantum yield of the zwitterionic o-TsABDI might be due to (1) the fluorescence that involves the low-probability π* → n charge transfer and (2) nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond.

A 2D covalent organic framework as a sensor for detecting formaldehyde.

Formaldehyde is the main cause of indoor pollution. In this research, we investigated the mechanism that the covalent organic framework (COF) identifies formaldehyde applying density functional theory (DFT) and time-dependent (TD) DFT approaches. On one hand, the calculation results of the geometric parameters, IR spectra, as well as 1H-NMR chemical shifts for protons that associated with the hydrogen bonding formation together with the electronic transition energies verified that the furcate hydrogen bonding formed between the COF and formaldehyde is enhanced in the excited S1 state and it is not beneficial to luminescence of the COF. On the other hand, excitingly, our further calculation results of the fluorescence rate coefficients also revealed that the strengthened hydrogen bonding behavior in S1 state caused an efficiently weakened luminescent phenomenon compared with that of the COF. Therefore, this analysis method, which qualitative collaborates with quantitative theoretically, demonstrates the possibility that the COF could be served as a sensor to detect formaldehyde. Graphical Abstract The luminescence of the Covalent Organic Framework can be greatly weakened after the addition of formaldehyde.

Spectral and photophysical properties of cytisine in acetonitrile – Theory and experiment

Spectral and photophysical properties of (−)-cytisine that is used as a smoking cessation aid, and which derivatives are promising tools in a treatment of neurological diseases, were investigated in acetonitrile, non-specifically interacting solvent with a polarity similar to water. The two chair conformers of cytisine were found the most stable in the ground state S0 and the lowest excited singlet state S1(π,π*), wherein axial one was characterized by a significantly larger abundance, fluorescence lifetime 0.15 ns and fluorescence quantum yield 0.008. The S1(π,π*) excited state of both cytisine conformers deactivated almost exclusively via internal conversion.

Insights into the unusual dual fluorescence of the ortho-amino analogue of green fluorescent protein chromophore

Even though both p-aminobenzonitrile (p-ABN) and o-ABDI [o-amino analogue of green fluorescent protein (GFP)] have the same electron-donating amino group, o-ABDI displays dual fluorescence while p-ABN exhibits single fluorescence. Even though both o-AABDI (o-acetamido analogue of GFP) and o-ABDI have the similar intramolecular hydrogen bonding, o-ABDI displays dual fluorescence while o-AABDI shows single fluorescence. To explore the unusual phenomenon of dual fluorescence for o-ABDI, we used the time-dependent density functional theory (TD-DFT) method with the polarizable continuum model (PCM) to study the S1 excited state of o-ABDI in acetonitrile. What we found is that o-ABDI in acetonitrile has two isomeric conformations (minima) in the S1 excited state, the twisted intramolecular charge transfer (TICT) and the localized excited (LE) states. The TICT state involves a large dipole moment change during fluorescence emission, while the LE state with a flat structure involves only a slight dipole moment change without a charge transfer during fluorescence emission. Even though both o-DMABDI (o-dimethylamino analogue of GFP) and o-ABDI have the TICT excited state, o-DMABDI does not have the stabilized LE S1 state while o-ABDI does. It is because the intramolecular hydrogen bonding of o-ABDI makes its LE S1 state highly stabilized, and that leads its LE S1 state to become a global minimum. That is why o-ABDI displays dual fluorescence.

Ultrafast X-ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics.

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B12, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S1 excited state structure within 500 fs of excitation.

Theoretical insight in highly luminescent properties of Eu(III) complex with phenanthroline

The complete photophysical process, from absorption in the UV region, excited state energy transitions, ligand → Eu(III) energy transfer to luminescence of Eu(phen)2(NO3)3 in Vis region, is elucidated by means of two theoretical approaches and in close relation to the experiment. The energy diagram, S1 and T1 excited state character and the concurrent energy transitions in phen and its Eu(III) complex in gas phase and in DMF solution are predicted by DFT-TDDFT/PBE1PBE calculations. The contributions of phen and NO3 ligands to the excited state energies are evaluated in detail. The high population of T1 state (due to effective intersystem crossing) and its closeness in energy to the 5D0 level of the Eu(III) (0.4 eV), make T1 state the key donor for the excited state ligand-to-metal ion energy transfer. The T1 state energy of phen chromophore determines this one in its Eu(III) and Gd(III) complexes. The solvent DMF effect on the lowest triplet excited state is insignificant. Providing similar accuracy for the vertical excited state energy as the estimates for the TDDFT/PBE1PBE energy of T1 minimum and the experimental phosphorescence data, INDO/S-CIS method could be reliably used for further prediction of the luminescent properties. A computational strategy based on Sparkle/RM1 geometry, INDO/S-CIS excitation energies, Judd-Ofelt parameters, obtained with the QDC model and energy transfer rates predicted T1 → 5D0 as the most probable channel of the energy transfer process and luminescent quantum yield of 32% close to the experimental result of 35%.

Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes.

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S1 state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S1 state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S0 state, but basically, the effect in both states remains the same. In the S0 and S1 states, the d(C-X) distances to the substituent's first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S1), LUMO(S1), and HOMO(S1)-LUMO(S1) gap correlate also with the pEDA(S1) and cSAR(S1) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S0) and HOMO(S1) orbitals shows that for case (i) HOMO orbitals exhibit almost perfect antisymmetry against the benzene plane, for case (ii) the antisymmetry of HOMO in one of the states is either perturbed or changed, and for case (iii) one HOMO state has σ-character.

Unraveling the electronic relaxation dynamics in photoexcited 2,4-difluoroaniline via femtosecond time-resolved photoelectron imaging.

Time-resolved photoelectron imaging is employed to investigate the relaxation dynamics of the lowest two excited electronic states S1(ππ*) and S2(π3s/πσ*) in 2,4-difluoroaniline (24DFA). As the S1(ππ*) state is populated directly following 289 nm excitation, the population undergoes ultrafast intramolecular vibrational redistribution on a 540 fs time scale, followed by efficient intersystem crossing from S1(ππ*) to the triplet state within 379 ps, and the subsequent slower deactivation process of the triplet state. For excitation to the S2(π3s/πσ*) state at 238 nm, the population probably bifurcates into two decay channels. The dominant channel with 84 fs involves ultrafast internal conversion to the S1(ππ*) state, from which it relaxes to the electronic ground state on a 116 ps time scale. The other appears to involve motion along the S2(π3s/πσ*) potential energy surface. Our data also determine experimentally the electronic energies of S2(π3s/πσ*), S3(ππ*), and several Rydberg states in 24DFA.

Pyrene and its selected 1-substituted derivatives revisited: A combined spectroscopic and computational investigation

Pyrene and its OH, NH2, and CN substituted derivatives were investigated experimentally by using UV/Vis absorption spectroscopy, steady state and time-resolved fluorescence spectroscopy in different polarity solvents. The computational part includes investigation of Pyrene and derivatives in excited S1 state in gas phase and in solution. Calculations were carried out with density functional theory (DFT) and time-dependent density functional theory (TDDFT) at B3LYP/6-311++G(d,p) level. Both ground and excited state geometries were fully optimized in gas phase and in solution. Solution calculations were carried out with Polarizable Continuum Model (PCM). Current results indicate that solvent polarity did not affect Py and its derivatives except PyNH2. Solvent has minor effects on PyCN and PyOH. Although PyNH2 has he smallest HOMO-LUMO energy gap, it is not the best candidate due to its shortest fluorescence lifetime. On the other hand, PyOH and PyCN have longer lifetimes. Therefore, it is concluded that investigated molecules are appropriate candidates for photosensitive applications in the order of PyNH2<PyOH < PyCN < Py.

The role of Herzberg-Teller effects on the resonance Raman spectrum of trans-porphycene investigated by time dependent density functional theory.

The S1 excited state properties as well as the associated absorption and resonance Raman (RR) spectra of trans-porphycene are investigated by means of time dependent density functional theory calculations. The relative magnitude of the Franck-Condon (FC) contribution and of the Herzberg-Teller (HT) effects is evaluated for both the absorption and RR intensities. The accuracy of the calculated spectra is assessed by employing different theoretical approximations and by comparing with experimental data. The obtained results show that Duschinsky effects lead to noticeable modifications in the absorption intensities but are nearly negligible in the RR spectrum. By contrast, the HT effects are stronger for the RR intensities compared to the absorption intensities, and these effects significantly improve the agreement with the experimental RR spectrum. Moreover, the HT effects produce different values of the RR depolarization ratios, which can be used to quantify the relative importance of the FC and HT contributions. Generally, it is found that the HT effects have a significant role on the RR spectrum of trans-porphycene and that their inclusion in the computational scheme is mandatory to accurately predict the RR intensities.

Excited-State Proton Transfer in Thiazolo-[4, 5-d]thiazo Heterocyclic Systems and the Geometry Alterations' Effect on Photophysical Characters: A Theoretical Study.

Thiazolo-[4,5- d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S1 PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH2) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S1 excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well.

How Methylation Modifies the Photophysics of the Native All- trans-Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol.

A comparison between the free-energy surfaces of the all- trans-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S1 excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck-Condon (FC) point. The conical intersections yielding the 9- cis and 11- cis isomers are stabilized while that yielding the 13- cis isomer is destabilized; in fact, it becomes inaccessible by excitation to S1. Furthermore, the planar S1 minimum is not present in the methylated compound. The solvent notably stabilizes the S2 excited state at the FC geometry. Therefore, if the S2 state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study.

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S1 and T1 excited state energetics and S1 ⇔ T1 intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Tuning the photophysical properties of carboranyl luminophores by closo- to nido-carborane conversion and application to OFF-ON fluoride sensing.

A family of closo-carborane-appended luminophores (closo-OXD1-2 and closo-DPS1-2) in which 2-R-o-carboranes (R = H, Me) are attached to the diphenyl-1,3,4-oxadiazole (OXD) or diphenyl-sulfone (DPS) acceptor groups were prepared and characterized. Deboronation of the closo-carborane cage produced the corresponding nido-carboranyl luminophores (nido-OXD1-2 and nido-DPS1-2). Whereas the closo-compounds were poorly emissive in THF (ΦPL < 0.01), the nido-luminophores exhibited an intense fluorescence with good quantum yields (ΦPL = 0.1-0.45). Electrochemical studies showed that while the closo-OXD and -DPS compounds displayed only carborane-centred, quasi-reversible reduction, the nido-compounds exhibited the typical features for nido-carborane-centred, irreversible oxidation and acceptor-centred, reversible reduction. Theoretical studies suggested that while the 1ππ* state of closo-compounds is nonemissive due to the contribution of closo-carborane to the LUMO in the S1 excited state, the intramolecular charge transfer (ICT) state from the nido-carborane to acceptor moieties in nido-compounds leads to an efficient fluorescence. Finally, THF solutions of closo-OXD1 and -DPS1 showed strong fluorescence upon the addition of fluoride anions under mild heating, but were intact to other anions, including cyanide, allowing the selective OFF-ON fluorescence sensing of fluoride.

Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface-Mounted Metal Organic Framework

Ordered porphyrin-based metal organic frameworks (MOFs) may serve as a model for mimicking the natural photosynthesis with highly ordered chlorophylls, i.e., porphyrin-like chromophores. Study of light harvesting and energy transfer as the primary event of photosynthesis is of great importance leading to improvement of photovoltaics overall performance. Detailed characterization of ultrafast dynamics of zinc tetraphenylporphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various steady-state and time-resolved laser spectroscopic techniques, i.e., time-correlated single photon counting, fluorescence up-conversion and transient absorption pump–probe with 20 fs resolution. Obtained results in these nanoporous materials were compared with corresponding results for ZnTPP in ethanol measured under the same conditions. Dramatic quenching of both upper excited singlet state S2 and first excited state S1 was observed. Subpicosecond and picosecond lifetimes were detected in transi...

Molecular engineering of the photo switching in the ortho chromophores of the nanostructured green fluorescence protein

Green fluorescent protein (GFP) has attracted wide attention as an efficient fluorescent probe for photophysical properties studies in the biological and biochemical sciences. In this work, the effect of different substituents (OMe, OEt, NO2, Br, CN, and CF3) in the para position of the phenyl ring in 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the GFP, on their photophysical properties were explored in both gas and solution phases by using TD-DFT method at PBE0/6-311++G(d,p) and M06-2X/6-311++G(d,p) levels of theory. The potential energy surfaces (PESs) were evaluated along the reaction coordinate (RC = dOH) at the ground S0 and excited S1 states of the three o-HBDI derivatives composed of electron donating and accepting substitutes in both gas and solution phases. The structural, electronic and photophysical properties, the atomic charges and electron density at critical points were examined. The results show the photophysical properties of o-HBDI are dependent on substitution patterns and solvents. In contrast to S0 state, the excited-state intramolecular proton transfer (ESIPT) process at S1 state in these compounds is expected to be approximately less barrier height. The ESIPT process in all three solvents is predicted to be an exergonic reaction. Depending on the strength of the electronic accepting or donating of the substituents, a wide tunable range of the fluorescence emissions from 460nm (visible blue light) to 642nm (visible red light) was predicted in the solvent media. By comparing wavelengths, the maximum value of the stoke shift is observed for CN-o-HBDI and TF-o-HBDI. Chromophores composed of the electron donating substituents (OMe and OEt) represent greatest red-shift in non-polar solvent cyclohexane. Our computed results are in good agreement with the experimental observations and have the potential applications in manipulating and tuning photo-physical properties of the technologically and biologically important fluorescent protein.

Vibronic relaxation dynamics of o-dichlorobenzene in its lowest excited singlet state

Vibronic dynamics of o-dichlorobenzene in its lowest excited singlet state, S1, is investigated in real time by using femtosecond pump-probe method, combined with time-of-flight mass spectroscopy and photoelectron velocity mapping technique. Relaxation processes for the excitation in the range of 276–252 nm can be fitted by single exponential decay model, while in the case of wavelength shorter than 252 nm two-exponential decay model must be adopted for simulating transient profiles. Lifetime constants of the vibrationally excited S1 states change from 651 ± 10 ps for 276 nm excitation to 61 ± 1 ps for 242 nm excitation. Both the internal conversion from the S1 to the highly vibrationally excited ground state S0 and the intersystem crossing from the S1 to the triplet state are supposed to play important roles in de-excitation processes. Exponential fitting of the de-excitation rates on the excitation energy implies such de-excitation process starts from the highly vibrationally excited S0 state, which is validated, by probing the relaxation following photoexcitation at 281 nm, below the S1 origin. Time-dependent photoelectron kinetic energy distributions have been obtained experimentally. As the excitation wavelength changes from 276 nm to 242 nm, different cationic vibronic vibrations can be populated, determined by the Franck-Condon factors between the large geometry distorted excited singlet states and final cationic states.